1. Field of the Invention
Apparatuses consistent with the present invention relate to a color-filterless LCD device which can be employed not only in small size LCD units but also in large size LCD units.
2. Description of the Related Art
Conventional liquid crystal display (LCD) devices include an LCD unit and a backlight unit which uniformly illuminates the LCD with white light. Since the LCD unit simply transmits or blocks the white light generated in the backlight unit, color filters transmitting red (R), green (G), and blue (B) light are needed to realize color in the LCD device. However, each color filter filters only one of the red, green, and blue light that is transmitted by the LCD unit, and therefore has a transmission rate of only approximately 30%. Taking into account the light loss of other optical components, only 10% of the light emitted from the backlight unit is actually transmitted, and most of the light loss is due to the color filters. In addition, color realization differs prominently according to the quality the color filter.
To solve the problem, a color-filterless LCD device which does not need a color filter to realize color is provided. FIG. 1 illustrates a conventional color-filterless LCD device. Referring to FIG. 1, the conventional color-filterless LCD device includes a backlight unit 20, which emits light at different exit angles corresponding to wavelengths of the light, and an LCD unit 10, which includes an optical device which focuses a light of a predetermined wavelength on a predetermined pixel.
The backlight unit 20 includes a transparent light guide plate 21 having a plurality of light sources 22 arranged on a lateral side thereof and a color separation sheet 23 disposed on an upper surface of the transparent light guide plate 21. The color separation sheet transmits light at different exit angles according to wavelengths of light. The light sources 22 can be a plurality of light emitting devices (LEDs) respectively emitting red, green, and blue light, or a plurality of LEDs emitting white light arranged in a row. The color separation sheet 23 may be a diffraction screen sheet formed of periodically arranged diffraction screens in a sine wave form, prism form, or quadrangles.
The LCD display unit 10 includes a cylinder lens 11, a transparent plate 12, a liquid crystal panel 13 having a liquid layer divided into a plurality of pixels, a diffraction optical device 14 which transmits light in a vertical direction, and a transparent plate 15.
In the conventional color-filterless LCD device, light incident on a lateral side of the light guide plate 21 is totally reflected inside the light guide plate 21, and is incident on the upper surface of the light guide plate 21 at an oblique angle. A portion of the light obliquely incident on the upper surface of the light guide plate 21 is totally reflected again, and a portion of the rest of the light is transmitted from the upper surface of the light guide plate 21 by the color separation sheet 23 with different exit angles according to wavelengths of the light. For example, the green (G) light exits at an exit angle of 0°, the blue (B) light exits at an exit angle of −7.8°, and the red (R) light exits at an exit angle of 10.5°. Then, the light is incident on the cylinder lens 11. As illustrated in FIG. 1, three cylinder lenses 11 are disposed to correspond to three pixels of the liquid crystal panel 13. The convergence location of the incident light on the cylinder lens 11 differs according, to the incident angle. Thus, as illustrated in FIG. 1, the green light is converged onto the pixel in the center, the blue light onto the pixel on the left in the drawing, and the red light onto the pixel on the right in the drawing. As each light of a different wavelength is incident on different pixels of the liquid crystal panel 13, color can be realized without color filters. Also, since light is incident on the liquid crystal panel 13 at an oblique angle, the light transmitted by the liquid crystal panel 13 is also emitted at an oblique angle. The diffraction optical device 14 emits light in a vertical direction and thus provides clearer images. The diffraction optical device 14 can be replaced with other optical devices having the same function, for example, a prism device.
As described above, such a conventional color-filterless LCD device does not need color filters and thus light loss is prevented. Accordingly, an LCD device with higher brightness is provided.
However, the color-filterless LCD devices in the prior art can only include edge type backlight units because the incident angle of incident light must be great enough for sufficient color separation by the color separation sheet 23. As illustrated in FIG. 2A, when an incident angle α of the incident light on the color separation sheet 23 is decreased, exit angles βB, βG, βR are increased and thus it is difficult to converge each color onto a desired location of a pixel in spite of color separation. To converge each color onto a desired location of a pixel, the exit angle of the green light is preferably 0°, and the red and blue light may have approximately symmetric exit angles to one another. For example, the red and blue light should have exit angles such as −10° for the red light and +10° for the blue light, or −8° for the red light and +8° for the blue light. For this purpose, it is advantageous that the incident angle α is as large as possible. For example, as illustrated in FIG. 2B, while dependant on the pattern of the color separation sheet 23, when the incident angle is approximately 70°, it satisfies the above described conditions. To obtain such a large incident angle, an edge type backlight unit was used for a conventional color-filterless LCD device. However, an edge type backlight unit can be used only in small size displays, and a direct type backlight unit is used in middle and large size displays. Accordingly, color-filterless LCD devices are employed in small size LCD devices.